In numerous applications, namely the machine tool, aeronautics and space industries, there is need for mechanisms for converting a high speed rotary motion, generally derived from an electric motor, into a relatively low speed, precise linear motion for the actuation of a control element. For example, when an internally threaded nut member is rotated by an electric motor at a conventional speed of 1800 rpm, then even with a thread pitch as fine as a 100 threads to the inch, a threaded shaft co-operating with the nut member will be nearly advanced by one inch for each 100 revolutions of the motor shaft. It would be highly desirable if the linear advancement of a shaft in response to a rotational input could be on the order of 1/100th or less that derived by utilizing a screw and nut arrangement. At the same time, the resulting mechanism should remain relatively simple and economical to manufacture by conventional machining techniques.